1. Field of the Invention
The invention relates to helicopter flight control systems. More particularly, the invention relates to a flight control system arrangement for a coaxial helicopter vehicle.
2. Description of Related Art
Coaxial helicopters have been known for many years. However, because of difficulties involved in the control of cyclic and collective pitch of rotor blades in a coaxial configuration, development of this type of aircraft has heretofore been limited. Conventional coaxial designs provide roll, pitch and yaw control by providing control input linkages for cyclic and collective pitch to both an upper rotor and a lower rotor of a coaxial rotor set. This has conventionally involved providing at least two swash plates; one below, and one above, the lower rotor, to transfer control inputs past the lower rotor to the upper rotor, which is rotating in the opposite direction.
Several successful coaxial designs have been developed, for example, those by Nikolai Kamov and the Kamov Design Bureau of the former Soviet Union. The Kamov organization continues to produce coaxial helicopters in the Russian Federation. Other coaxial designs exist, for example a small coaxial pilotless craft developed by United Technologies Corporation of Hartford, Conn. An example of the control system for this latter craft is disclosed in U.S. Pat. No. 5,058,824.
Coaxial designs are advantageous because they eliminate need for a tail rotor, and are generally more efficient. With a coaxial design, one way of providing yaw control is to provide a differential collective blade pitch control. Pitch is increased in one rotor, and decreased in the other, to unbalance torque. Another way of providing yaw control is to place one or more airfoils in the rotor set downwash. The airfoils are tiltable with respect to the downwash. The airfoils, nominally set to provide minimal air resistance in the downwash, intercept and redirect the downwash from the rotor set by tilting in one direction or the other from this initial position. This creates a reaction force vector at a location away from a yaw axis of rotation of the airframe; and tends to yaw the airframe right or left depending on which way the airfoils are tilted. An example of such a system is disclosed in U.S. Pat. No. 5,791,592, issued Aug. 11, 1998 to Nolan, et al. In the Nolan system there is no cyclic blade pitch control, as pitch and roll control are provided by tilting the rotor set with respect to the airframe; thus deflecting the thrust vector from the rotor set with respect to the airframe to pitch and roll the aircraft.